Advances in Fingerprint Technology.pdf

Figure 5.4 Formation of murexide (VI) in the reaction between alloxan (V) and
amino acids.
are sprayed with a concentrated solution of ninhydrin. The mechanism of
this reaction remains unclear. 22
A comprehensive review of ninhydrin and its chemical and physical
properties was published by McCaldin in 1960. 20 Of the two more recent
reviews on ninhydrin and related compounds, the one by Joullié et al. is of
particular importance because it covers not only general chemistry, but also
forensic aspects. 23,24 A comprehensive review of vicinal polycarbonyl compounds
has appeared very recently. 25
Reaction with Amino Acids
Ruhemann discovered and correctly interpreted ninhydrin’s most useful reaction
with alpha amino acids. 18 He was impressed by the close similarity
between ninhydrin and another cyclic triketone, alloxan, whose reaction with
alpha amino acids gives carbon dioxide, an aldehyde, and a blue compound,
murexide (Figure 5.4). Ruhemann showed that the purple product of the
ninhydrin reaction (later named after him, Ruhemann’s purple) was the
ninhydrin analogue of murexide. 17,18,26-28
Despite the fact that the ninhydrin reaction has been used extensively to
detect and estimate amino acids, its mechanism has not been fully understood
until recently; hence, it has given rise to a number of theories. There
was even a controversy regarding the type of amino acids that undergo this
reaction. Although Ruhemann claimed that not only alpha, but also beta
amino acids, give the purple-blue color when treated with the reagent, it has
been stated by others in later years that only alpha amino acids are reactive
in this manner. 29,30 The general nature of the reaction, however, gave an
indication of its probable course: if the reaction is as general as stated, it is
likely that the purple color is the same for all amino acids and that only a
fragment of the amino acid involved is contained in the colored compound. 20
A detailed discussion of the mechanism is beyond the scope of this chapter.
The currently acceptable general mechanism for the ninhydrin reaction,
as suggested by Friedman and Williams in 1974, 31 is outlined in Figure 5.5.
Ninhydrin (I) tautomerizes to 1,2,3-indanetrione (IV), which forms a Schiff’s